The primary objectives of the proposed research are (1) to separate the effects of red cell aggregation and red cell rigidity using a new filtration technique developed in this laboratory (using red cell suspensions in plasma and in buffered saline), and (2) to measure the changes in microcirculatory flow produced by each of these measured alterations in blood rheology. Changes in flow resistance and distribution of reduced velocities will be measured both in a branching segment of the rat mesentery microvasculature and in clear plastic models fabricated by the gallium injection technique. Red cell velocities in the mirocirculation will be measured using the "dual-slit" technique, and proximal pressures will be measured simultaneously from the feeding syringe. The effects of both hardened red cells and enhanced aggregation on microcirculatory flow will be studied.